SURABAYA, Indonesia - Suspected militants on a motorcycle detonated explosives outside police headquarters in Indonesia's second-largest city on Monday, a day after coordinated suicide bombings on three churches in the city killed at least eight people.

Police spokesman Frans Barung Mangera said six civilians and four police officers were wounded in the explosion at the police complex in Surabaya. The blast came just hours after police said one family, including girls aged 9 and 12, had carried out the church bombings.

The flurry of attacks have raised concerns that previously beaten down militant networks in the world's most populous Muslim-majority nation have been reinvigorated by the return of some of the estimated 1,100 Indonesians who went to fight with the Islamic State group in Syria.

Experts have warned for several years that when those fighter ret urn, they could pose a significant threat.

Police say the family that carried out Sunday's suicide bombings had indeed returned to Indonesia from Syria, and IS claimed responsibility for those bombings in a statement carried by its Aamaq news agency.

CCTV footage of the attack on Surabaya's police headquarters showed at least one explosion after two motorcycles, each with two people aboard, drove into a security checkpoint. The motorbikes, which moved closely together, pulled up alongside a car and four officers manning opposite sides of the checkpoint.

Two men, apparently civilians, were walking into the checkpoint just meters from the motorbikes at the moment of the explosion, which a split second later was followed by a second possible blast.

Indonesia's president Joko "Jokowi" Widodo condemned the string of attacks as "barbaric" and vowed that authorities would root out and "destroy" Islamic militan ts.

National police chief Tito Karnavian has said the father of the family that carried out the church bombings was head of the Surabaya cell of Jemaah Anshorut Daulah, an Indonesian militant network affiliated with IS that has been implicated in attacks in Indonesia in the past year. All six members of the family were killed.

The IS statement claiming the attacks didn't mention anything about families or children taking part and said there were only three attackers. The group also claimed responsibility for a hostage-taking ordeal last week by imprisoned Islamic militants at a detention centre near Jakarta in which six officers were killed.

Separately on Sunday, three members of another family were killed when homemade bombs exploded at an apartment in Sidoarjo, a town bordering Surabaya, police say.

Indonesia's deadliest terrorist attack occurred in 2002, when bombs exploded on the tourist island of Bali, killing 202 people in one nigh t, mostly foreigners.

The church attacks occurred within minutes of each other, according Mangera.

Karnavian said the father drove a bomb-laden car into the city's Pentecostal church. The mother, with her two daughters, attacked the Christian Church of Diponegoro, he said. Based on their remains, Karnavian said the mother and daughters were all wearing explosives around their waists.

The sons aged 16 and 18 rode a motorcycle onto the grounds of the Santa Maria Church and detonated their explosives there.

The fact that children were involved in Sunday's attacks has shocked and angered the country of more than 260 million people.